Research on geology, geophysics, and
petrology of impact structures (meteorite impact craters)
STOP 5: Megabreccia (near Belchite)
The megabreccia of the Azuara structure is a structural peculiarity which may be important for understanding some impact-cratering aspects. The breccia is exposed in the region of the Mesozoic outer ring where it concentrates in the north-east and north-west. Its name refers to the thickness (up to 80 m), the extension (several km˛), and the maximum size of the components (of the order of 20 m).
The megabreccia is a polymict breccia and occurs as a roughly stratiform layer in the sequence Carniolas (= Keuper/Liassic transitional layers) - megabreccia - Liassic (Fig.9). The thickness is variable between few meters and about 80 m. The components have resulted predominantly from in-situ brecciation of Carniolas and Liassic limestones and dolostones; however, rocks of stratigraphically adjacent layers are frequently intermixed. Occasionally, blocks of stratigraphically much younger (e.g., Eocene) sediments (red shales) contribute to the breccia, as well as white gravelly carbonate rocks of unknown stratigraphic origin.
The clast size ranges from a fraction of a millimeter to the order of 20 meters. In the largest blocks embedded in the matrix, the layering may widely be preserved. Neighboring blocks, however, are displaced, rotated or inverted. Strongly folded layers may heavily interlace with the matrix. Mortar texture and breccia generations (breccias-within-breccias) are abundant. Breccia dikes cutting across the megabreccia are mainly of the polymict type.
In thin section, cataclastic flow texture of the fine-grained carbonate matrix can be observed.
The megabreccia causes a strange landscape, which is characterized by a hummocky vegetationless surface and isolated large breccia megablocks emerging from the ground. On blowing the surface of the megabreccia with a hammer, brecciation and frequent hollows produce a characteristic dull and muffled tone. Therefore, the natives name the megabreccia landscapes „los montes vasios" ("the hollow mountains").
For more details, we refer to the work of Fiebag (1988) and Katschorek (1990).
Interpretation and relations
In the impact literature, the term megabreccia is used for extended breccia deposits with clasts of very large size. Megabreccias have been described for the Wells Creek (Stearns et al. 1968), Steinheim basin (Reiff 1977), Gosses Bluff (Glikson (1969), Wetumpka (Neathery et al. 1997), and other impact structures. The Alamo megabreccia (Warme & Sandberg 1996) as a record of a Late Devonian impact in southern Nevada is also well known.
In the past, the Azuara megabreccia has erroneously been mapped as Carniolas and Miocene, and the brecciation has been confused with the typical Carniolas rauhwacke brecciation well known from other parts of the Iberian Chain.
However, the occurrence of the Azuara megabreccia in an area obviously limited to the inner rim region in the north of the impact structure, the polymict character, and the incorporation of the Liassic prove an origin as a particular geological unit related with the impact process.
We suggest an origin of the megabreccia in the modification stage of impact cratering. There are models assuming that the modification stage already begins before the excavation has come to an end (Gault &Greely 1978, Croft 1981, and others). Accordingly, excavation flow and collapse/rebound processes may act simultaneously, and relative movements and countermovements of adjacent, stratiform beds may have produced the peripheral immense internal megabrecciation (Fig.9). We also suggest that the concentration of the megabrecciation on Carniolas and Liassic rocks is not at random. Both units are deposited on top of 50 - 200 m thick Keuper marls, which is the only incompetent unit of some thickness within the Azuara Mesozoic stratigraphy. The influence of soft layers on impact cratering may be important!
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